Sampling time of PCD2/3.W6x5 modules

FAQ #100395

In contrast to the conventional analogue modules (e.g. the W600), the DAC (Digital to Analogue Convertor) of the PCD2/3.W6x5 is not accessed directly over the internal I/O bus of the PCD. The command is first interpreted by the module's micro controller and then cyclically passed to the DAC.


Reason for the new function principle
The employment of a µC on the module can decreases the load of the PCD's main CPU because the conversion and linearization of the output value is done directly on the I/O module (before writing it to the DAC).
Because of the fact that the modules are equipped by a µC, they are called "intelligent analogue modules".


Handling by the user program
The command sequence for an analogue output value is too long for writing directly onto the internal I/O bus. Therefore specific FBoxes were created (also handling the module initialisation, value range checking and the interpretation of possible error flags from the module). The execution time of the FBox on a PCD2.M170 varies from 0.8 ms to 3.8 ms (depending on the stretch factor). The output value will be overtaken by the module's µC at the very ending of the execution of the code on the PCD.

Cycle time on the module
The µC of the modules PCD2/3.W605, PCD2/3.W615 and PCD2/3.W625 executes the following tasks cyclically:

  • Comparison whether new values were sent by the PCD

  • Calculation of the digital values to be sent to the DAC out of the value in user units

  • Writing the digital values to the DAC

The µC cycle time of the W6x5 modules is firmware depending. While it is around 4 ms with FW version 020, it is around 2 ms with FW version 021 and above. (The module's firmware version is printed on a sticker on the module, holding 3 digits)
Since the cycle time of the PCD main CPU and the µC of the W6x5 are asynchronous, it may take up to twice the cycle time from the time the value was sent to the module until it written to the DAC. Also consider the output filter that delays the real output signal by some more µs.
The communication between the internal PCD I/O bus and the µC of the module influences the cycle time only slightly.

The screenshot below visualizes the execution time of the FBox part of one Channel (upper graph) and the output signal of the W6x5 (lower graph, 2...3.7 ms after executed command on the PCD). This graphs were recorded with a PCD2.M170 and a PCD2.W625 FW 021.

Note regarding the TI
In the TI (up to version 5), a sampling rate of 2 ms is declared. This is not correct for modules equipped with FW version 020.
In the german version of the TI, a vaque translation of the sampling rate (translated to "Gesamtwandlungszeit") can cause missinterpretation.


Related Files:


PCD2 / Wxxx

PCD3 / Wxxx

Last update: 11.05.2015 12:04

First release: 07.07.2005 08:05

Views: 2226

The requested software / document is no longer marketed by Saia-Burgess Controls AG and without technical support. It is an older software version which can be operated only on certain now no longer commercially available products.